In recent years, the applications of storage batteries have been diversified to include mobile devices, automobiles, stationary power sources, and the like. Therefore, the development of next-generation secondary batteries which are inexpensive and have high energy densities has been expected as alternatives to conventional lithium ion secondary batteries. Among next-generation secondary batteries, magnesium secondary batteries are particularly advantageous in many aspects including the following: (i) high capacity can be expected because the two-electron reaction can be used for charging and discharging; (ii) magnesium that can be used for a negative electrode is excellent in safety and has a relatively low potential, thereby allowing high voltage operation of batteries; and (iii) magnesium is inexpensive with fewer risks of maldistribution of production areas. Therefore, the research and development of magnesium secondary batteries are in progress.
At the beginning of the development, TiS2, ZrS2, RuO2, CO3O4, V2O5, or the like was used as a positive electrode active material for a magnesium secondary battery. In recent years, magnesium composite oxides having various crystal structures have been proposed.
For example, Ichitsubo et al. discloses Mg0.67Ni1.33O2 having a rock salt-type structure as a positive electrode active material for a magnesium secondary battery (Tetsu Ichitsubo, et al., Journal of Materials Chemistry, 21, 11764 (2011)).
In addition, Yagi et al. discloses a method of synthesizing MgNiO2 having a rock salt-type structure (Shunsuke Yagi, et al., Japanese Journal of Applied Physics, 52, 025501 (2013)).